The present invention relates generally to implantable medical devices for providing stimulating pulses to selected body tissue, and more particularly, to the lead assemblies connecting such devices with the tissue to be stimulated.
Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing, or for sensing electrical signals produced by the heart and representing cardiac activity, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end a tip electrode designed to intimately contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end and the electrode at the distal end.
To prevent displacement or dislodgment of the tip electrode and to maintain the necessary stable electrical contact between the tip electrode and the endocardial tissue, the electrode must be firmly anchored relative to the tissue. A number of methods, both passive and active, have been devised for this purpose. In accordance with one known passive fixation technique, a plurality of flexible tines are molded integrally with the insulative sheath covering the coiled electrical conductors and extend rearwardly at an acute angle relative to the longitudinal axis of the lead. Following implantation of the lead, the tines become entangled in the trabecular network thereby securing the electrode position. Since the tines can flatten against the lead body and thus reduce its diameter, tined leads are often suitable for introduction through small blood veins. Other known passive fixation techniques include collar electrodes which have one or more conical projections of silicon rubber or other biocompatible flexible material behind the electrode tip. Like the tines, the cone becomes entangled in the trabecular network inside the heart, thereby anchoring the electrode tip. In yet another known approach which is advantageous if relocation of the electrode tip becomes necessary, projecting, flexible fins are used to provide stable anchoring. It is known, however, that even small displacements or xe2x80x9cmicrodisiodgmentsxe2x80x9d of the tip electrode can result in sporadic losses of reliable electrical contact between the tip electrode and the tissue engaged thereby resulting in increases in the stimulation voltage.
The optimization of the design of a passive fixation tip electrode depends on many factors. If the surface area of an electrode is too great too much current is drained from the battery due to low impedances. Alternatively, if the electrode area is not great enough excessive voltage is necessary in order to deliver the proper current flux density required for stimulation. An optimal electrode in terms of surface area minimizes both the voltage and current required for stimulation. Similarly, shape plays an important role in the determination of the effectiveness of the electrode. An optimized electrode shape, for a given electrode surface area, minimizes the current and voltage required for stimulation while providing the necessary current flux density for stimulation.
The presence of a steroidal anti-inflammatory, such as dexamethasone, is known to reduce the threshold for stimulation by minimizing fibrotic encapsulation or fibrosis which occurs toward the end of any normal healing response to the implant of an electrode. This minimization of fibrosis allows the electrode and excitable tissue to be in closer proximity, the result of this suppressed foreign body reaction being lower voltage and current requirements at threshold. Thus, steroid eluting tip electrodes can be made smaller than their nonsteroid counterparts and therefore will present higher pacing impedances and afford lower voltage thresholds, minimizing current drain and preserving battery life. Such electrode structures are disclosed, for example, in U.S. Pat. Nos. 5,282,844 and 5,408,744.
Despite the advances that have been made in this field, there remains a need for lead assemblies having tip electrode structures that minimize both the voltage and current required for stimulation so as to preserve battery life.
In accordance with one exemplary embodiment of the present invention, there is provided a passive fixation, body implantable lead assembly adapted to transmit electrical signals between a proximal end portion of the lead assembly and a distal end portion of the lead assembly and to thereby stimulate selected body tissue and/or sense electrical signals therefrom. The lead assembly has a longitudinal axis and comprises an electrical conductor extending between the proximal and distal end portions of the lead assembly for transmitting the electrical signals, a sheath of insulative, biocompatible, biostable material enclosing the electrical conductor, and a tip electrode electrically connected to the distal end of the electrical conductor. In accordance with a preferred form of the invention, the tip electrode is in the form of a cylinder disposed coaxially of the longitudinal axis of the lead assembly, and has a side surface and a distal extremity. The distal extremity of the tip electrode comprises a generally planar, disk shaped active electrode surface extending substantially perpendicular to the longitudinal axis of the assembly. Last, a thin dielectric insulating layer covers substantially the entire side surface of the tip electrode. By so masking the active electrode surface of the tip electrode with a dielectric insulator, the pacing impedance and current density of the electrode may be substantially increased. Further improvements in this connection may be achieved by reducing the active electrode surface area to less than about 1 mm2.
In accordance with another aspect of the invention, the side surface of the tip electrode includes a distal portion and a proximal portion and a drug dispensing member is disposed around the proximal portion of the tip electrode for storing a drug to be dispensed to the body tissue. Such a drug dispensing member preferably takes the form of a collar adapted to be loaded with, for example, a steroidal anti-inflammatory such as dexamethasone which serves to reduce the stimulation threshold by minimizing fibrosis.
In accordance with yet another aspect of the present invention, the sheath may include a first set of anchoring tines extending outwardly from the sheath, the first set of tines being disposed adjacent the distal extremity of the sheath. A second set of anchoring tines extends outwardly from the sheath and away from the tip electrode to form an acute angle with the sheath material, the second set of tines being disposed proximally of the first set tines along the distal end portion of the lead assembly. The tines of the first set of tines have a length less than that of the second set of tines. By increasing the surface area of the distal portion of the lead assembly engaging the trabecular network inside the heart, the xe2x80x9cnubbyxe2x80x9d first set of tines tends to increase the stability of lead fixation to avoid microdislodgments of the tip electrode which may cause sporadic increases in stimulation voltage.